1. Field of the Invention
The present invention relates to methods for preparing sputter target assemblies, and to the sputter target assemblies prepared by these methods. In particular, the invention relates to pot-shaped, or hollow cathode, sputter target assemblies, and methods for making such assemblies.
2. Description of the Prior Art
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer or film on the receiving surface of the substrate.
The sputter target is heated during the sputtering process by the thermal energy of the bombarding gas ions. In conventional cathode target assemblies, the target is attached to a nonmagnetic backing plate. The backing plate is typically water-cooled to carry away the heat generated by the ion bombardment of the target.
One type of knows system for coating substrates by cathodic sputtering includes a cathode having a hollow body with an open end facing the substrate and anode. A series of magnets may be provided around the hollow cathode body for optimizing the ionization through magnetic fields. As such, coating coverage and uniformity on the substrate is improved. Examples of such hollow cathode assemblies are illustrated in U.S. Pat. No. 5,985,115 to Hartsough et al. U.S. Pat. No. 4,966,677 to Aichert et al. and U.S. Pat. No. 5,728,280 to Scherer, both of which are incorporated herein by reference.
High purity metal and metal alloys having a purity level greater than that available from commercial grade materials and typically selected form the group including titanium, copper, tantalum, cobalt, tungsten and aluminum, are utilized as the sputtering material in conventional target cathode assemblies. As may be appreciated, such high purity metals and metal alloys are often not readily available and are costly to obtain. Additionally, typical target cathode assembly sputtering materials have relatively high specific gravities resulting in difficulty in their manipulation due to excessive weight.
The above described traditional hollow cathode target assemblies are monolithic in design such that the assembly is formed entirely of the costly and often relatively heavy high purity metals and metal alloys. Additionally, such hollow cathode target assemblies are generally inefficient in that a significant portion of the sputtering material remains unused when the assembly requires replacement.
Accordingly, there remains a need for a hollow cathode target assembly which increases the efficient use of the expensive high-purity sputtering material and generally reduces the assembly""s overall weight. Additionally. there is a need for a method of producing such a hollow cathode target assembly.
The present invention provides a two-piece pot-shaped, or hollow cathode, sputter target assembly including a sputtering insert of high purity sputtering material which is concentrically received with an outer shell of less expensive, lower purity and preferably, lighter weight material.
More particularly, the invention contemplates a sputter target assembly comprising a sputtering insert including a substantially cylindrical side wall defining a longitudinal axis and having inner and outer surfaces. One end of the 30 sputtering insert is closed by a substantially planar end wall connected to the side wall and having an inner surface connected to the inner surface of the side wall The opposing end of the sputtering insert is open such that the inner surfaces of the side wall and end wall define a cup-shaped sputtering surface.
The sputtering insert is concentrically received within an outer shell. The outer shell includes a substantially cylindrical side wall having inner and outer surfaces. A substantially planar end wall is connected to the side wall at one end of the outer shell and includes an inner surface. The inner surfaces of the side wall and end wall of the outer shell are adapted to mate with the outer surfaces of the side wall and end wall of the sputtering insert along an interfacial area located between the sputtering insert and the outer shell. The interfacial area includes a substantially cylindrical portion extending coaxial to the longitudinal axis.
The sputtering insert is preferably composed of a first metallic material selected from the group consisting of high purity titanium, copper, tantalum, cobalt, tungsten, aluminum and alloys thereof. The outer shell is preferably composed of a second metallic material selected from the group consisting of low purity aluminum, copper, steel, titanium and alloys thereof. The first metallic material has a purity significantly greater than that of a respective second metallic material.
One embodiment of the method of the present invention for forming the above-described two-piece hollow cathode sputter target assembly includes the steps of forming a blank of a first metallic material into a sputtering insert including a substantially cylindrical side wall and an end wall, the side wall and the end wall defining an outer mating surface and an inner sputtering surface. A blank of a second metallic material is formed into an outer shell including a substantially cylindrical side wall and an end wall, the side wall and end wall defining an inner mating surface.
The sputtering insert is positioned concentrically within the outer shell and a substantially cylindrical plug is next positioned concentrically within the sputtering insert. The sputtering insert and plug are then placed within a hot isostatic press can. A closure plate is then secured to the can to form a vacuum tight can assembly wherein residual air is evacuated from the can assembly. The can assembly is then subjected to a predetermined temperature at a predetermined pressure for a predetermined period of time, thereby diffusion bonding the sputtering insert to the outer shell.
A first alternative embodiment of the method of the present invention includes the steps of forming the outer shell and sputtering insert as detailed above but in a manner providing for an interference fit at room temperature. More particularly, the inner diameter of the side wall of the outer shell is selected to be less than the outer diameter of the sputtering insert at ambient room temperature. The sputtering insert is preferably provided at a first predetermined temperature no greater than ambient room temperature and the outer shell is heated to a second predetermined temperature above ambient room temperature. Next, the sputtering insert is slidably received within the outer shell. The sputtering insert is then heated, if necessary, and the outer shell cooled, to ambient room temperature wherein the outer shell contracts, thereby providing an interference fit between the side walls of the sputtering insert and the outer shell at room temperature.
In a second alternative embodiment of the method of the present invention, a substantially planar sputtering blank composed of a first metallic material having a first mating surface is provided. Likewise, a substantially planar shell blank composed of a second metallic material having a second mating surface is provided. The shell blank is bonded with the sputtering blank to form a blank assembly.
Bonding is preferably accomplished by pressing the first mating surface together with the second mating surface at a predetermined temperature below melting points of the first and second metallic materials such that a diffusion bond is formed along the first and second mating surfaces to define a blank assembly. The blank assembly is next formed using traditional metal working methods into a cup-shaped sputter target assembly including an outer shell composed of the shell blank and a sputtering insert composed of the sputtering blank wherein the sputtering insert is concentrically disposed within the outer shell. The sputter target assembly includes an outer cylindrical wall defined by the outer shell and an inner cylindrical wall defined by the sputtering insert wherein the inner cylindrical wall is bonded to the outer cylindrical wall.
Therefore, it is an object of the present invention to provide a hollow cathode target assembly which increases material efficiency by reducing the amount of expensive sputtering material which is not fully processed.
It is a further object of the present invention to provide a hollow cathode target assembly of a reduced weight, thereby facilitating manipulation thereof.
It is another object of the present invention to provide a hollow cathode target assembly which is inexpensive.
It is yet another object of the present invention to provide a method of forming such a hollow cathode target assembly by providing a relatively inexpensive outer shell which concentrically receives a high purity sputtering insert.
It is a further object of the present invention to provide such a method of forming a hollow cathode target assembly which provides adequate bonding strength.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.